Planar chromatographies

In practice, the majority of TLC separations are qualitative or semiquantitative (visual comparison) in nature. However, modern computer-controlled densitometers are now available that scan sample and calibrator chromatograms in tracks on HPTLC plates and provide quantitative capabilities. Clinically relevant analytes that have been measured by TLC include amino acids, bile acids, carbohydrates, drugs, fipids, glycolipids, phospholipids, porphyrins, prostaglandins, steroid hormones, purines, pyrimidines, derivatives of nucleic acid, and urinary organic acids. The advantages of TLC include simphcity, rapidity, versatility, ability to process a large number of samples 

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EXPERIMENTAL UNITS FOR PLANAR CHROMATOGRAPHY WITH EXTERNAL CONTROL PROPARTIES OF THE CHROMATOGRAPHIC SYSTEM... 

Geiss, F. Fundamentals of Thin Layer Chromatography (Planar Chromatography). Huthig-Verlag, Heidelberg-Basel-New York 1987. 

Kunz, F. R., Jork, H. Proceedings of the 4th International Symposium on Instrumental TLC (Planar Chromatography), Selvino. Bad Diirkheim IfC-Verlag 1987, S. 437-451. 

Two-dimensional separations can be represented on a flat bed, by analogy with planar chromatography, with components represented by a series of dots . In fact, zone broadening processes in the two dimensions result in elliptically shaped spots centred on each dot . Overlap of the spots is then possible, but Bertsch (30) also showed how the contributors to the overall resolution, R, along the two axes, and Ry contribute to the final resolution according to the following ... 

The main origin of multidimensional chromatography lies in planar chromatography. The development of paper chromatography, i.e. the partition between a liquid moving by capillary action across a strip of paper impregnated with a second liquid... 

D TLC combined with multiple development is therefore a promising route which should lead to real improvements in planar chromatography in the near future. 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

Theoretically, 3-D OPLC in combination with multiple development is the most powerful technique of instrumental planar chromatography. Unfortunately, suitable instrumentation is at an early stage of development. 

Chromatographic plates can be connected for both capillary-controlled and forced-flow planar chromatography (FFPC), i.e. irrespective of whether capillary action or forced-flow is the driving force for the separation. The first technique is denoted as grafted planar chromatography (31), while the second is known as long distance (LD) OPLC, which uses heterolayers (32, 33). 

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

SERIALLY CONNECTED MULTILAYER FORCED-FLOW PLANAR CHROMATOGRAPHY... 

On the basis of theory and experimental observations it can be predicted that a zone capacity of ca. 1500 could be achieved by 2-D multiple development. Because the same result can be achieved by application of 2-D forced-flow development on HPTLC plates, it can be stated that the combination of stationary phases, FFPC and "D offers a fruitful future in modem, instmmental planar chromatography. 

Sz. Nyiredy, K. Dallenhach-Toelke and O. Sticher, The PRISMA optimization system in planar chromatography , /. Planar Chromatogr. 1 336-342 (1988). 

Sz. Nyiiedy, Planar chromatography . Chromatography, E. Fleftmann (Ed.), Elsevier, Amsterdam, A109-A150 (1992). 

Sz. Nyiredy, Zs. Fater and B. Szabady, Identification in planar chromatography by use of retention data measured using characterized mobile phases , ]. Planar Chromatogr. 7 406-409 (1994). 

Often, planar chromatography is used as a preparative step for the isolation of single components or classes of components for further chromatographic separation or spectroscopic elucidation. Many planar chromatographic methods have been developed for the analysis of food products, bioactive compounds from plant materials, and essential oils. 

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